The present invention relates generally to an image display system, and more particularly to an optical system for a reflection type display device-incorporating display system that is compact, portable, and low power consumption.
Among small-format display systems, there are cellular phones and personal digital assistants. For these small-format display systems, liquid crystal display devices of the direct-view type are often used. Especially for high-definition, high pixel resolution images and moving images, it is required to make use of active matrix liquid crystals having fast display speeds. However, the active matrix liquid crystals cost much and so do systems. The active matrix liquid crystals consume much power, and require large-capacity batteries for displays over an extended period of time. Further, there is an apprehension that images appearing on image display systems may be peeped by those around the viewer.
On the other hand, there are small-format display device-incorporating display systems wherein an image appearing on a display device is magnified and displayed through an optical system, as set forth typically in JP-A's 48-102527 and 5-303054. In these display systems, an image appearing on the display device is magnified and displayed as a virtual image using a concave mirror. The latter in particular makes use of a non-rotationally symmetric reflecting surface whereby an image with reduced aberrations is obtained. However, the size of the display device must be relatively large; it is not always possible to rely on a display device that is much smaller than that used with a direct-view type display system.
An approach involving the projection midair of an image on a display device through a projection optical system and the magnification and display of that image by a concave mirror is disclosed in JP-A's 5-303055 and 2000-221440, and systems therefor are shown in JP-A's 7-270781 and 9-139901.
Furthermore, JP-A 2002-268005 comes up with a small-format, low power consumption display system wherein an image appearing on a display device or its intermediate image is projected through a relay optical system that comprises a decentered prism optical system. A light beam from that relay optical system is then converged to the eyeball of a viewer through an eyepiece optical system. In this display system, an image projected through the relay optical system is formed near the eyepiece optical system, and the exit pupil of the relay optical system is projected onto the viewer's eyeball.
It is here noted that a reflection type liquid crystal display device is higher than a transmission type liquid crystal display device in terms of aperture efficiency. This is because in the transmission type liquid crystal display device a drive circuit comprising interconnection wires, electrodes, etc. is located within a display screen surface whereas in the reflection type liquid crystal display device these are mounted on the back surface. For this reason, the reflection type liquid crystal display device can present bright image displays. The reflection type liquid crystal display device, because of being capable of achieving high densities, can provide a display system having high-pixel resolution. Another possible reflection type display device is typically a DMD (digital micro-mirror device).